Millimetre waves are radio waves in the wavelength range from 1 mm to 10 mm, which corresponds to a frequency range from 30 GHz to 300 GHz. These waves have been applied to in-vehicle radar devices for measuring the distance between moving vehicles in order to prevent collisions and to high speed wireless communications for achieving transmission data rate in the order of several gigabits per second. Further, improved generation and detection techniques as well as latest improvements in the integration and miniaturization of devices operating in the millimetre wave frequency range have created a lot of interest to exploit the properties of this electromagnetic radiation. Its ability to penetrate non metal materials, including plastics, walls, clothes, smoke and fog have provided additional momentum to research on millimetre waves imaging applications. Furthermore, the achievable spatial resolution of few millimetres is very interesting in security applications like concealed weapon or explosives detection.
The millimetre waves can be used in an active or a passive way. A passive radiometric imaging system creates images by capturing the electromagnetic radiation emitted by the objects by using a radiometer. Such a passive radiometric imaging system is, for instance, known from US 2007/0221847 A1. An active radiometric imaging system, on the other hand, irradiates millimetre waves to objects and creates images by detecting the transmitted or reflected waves. Millimetre waves do not produce ionizing effects, which makes this technology an attractive candidate to be employed in security applications like concealed weapon or explosives detection (existing technologies based on infrared or visible radiation can not detect concealed objects, and X-ray based systems can not be used in humans due to its ionizing effect).